MPEG2 is a standard which has been proposed for the digital encoding of television signals. MPEG2 allows for signals to be encoded on either an interlaced or progressive basis.
The term interlaced is used to refer to image data that is represented using, e.g., two alternating image fields. One field of an interlaced video frame normally corresponds to the odd lines of pixels in the frame with the other field corresponding to the even lines of the frame. During display, the lines of one field are scanned, e.g., output to a display device. The lines of the second field are then scanned, so that they are interlaced on the display device with the lines of the first field. In this manner, when interlaced images are displayed, odd and even lines of image data are updated on an alternating basis.
In the case of progressive image data, image data is displayed sequentially, e.g., starting at the top left corner of an image proceeding to the bottom right corner. Thus, in the progressive image case, the lines of an image are displayed or updated on a sequential basis without lines being skipped.
In a series of progressive images, the positioning of the horizontal display lines is consistent from image to image. Accordingly, each newly displayed progressive image normally will completely replace the previous image on a display assuming that the images are the same size. In the case of interlaced images, each frame includes two fields which correspond to spatially different, e.g., horizontal odd and even lines, of a display device. Accordingly, in the case of interlaced images, each field updates only a portion of the displayed image. Because fields of an interlaced image normally correspond to images at different times, merely combining fields 1 and 2 of an interlaced frame can cause blurring and other image distortions when motion is present. For this reason, conversion of interlaced images to progressive images normally involves some form of motion detection and the application of processing which is a function of detected motion.
Presently television images are usually encoded to be displayed as interlaced images. Unfortunately, most computers are designed to display progressively scanned images.
The ability to efficiently convert between interlaced and progressive image formats continues to increase in importance due, in part, to the ever increasing use of computers. Notably, when television scenes or other data represented as interlaced images are to be displayed on a computer, they normally first have to be converted into progressive image data.
High speed memory is normally used in video processing applications which convert between image formats. This is so that real time, or near real time, processing of video data can be achieved. While the cost of memory has dropped considerably in recent years, memory still remains a significant cost component of many image and video processing systems. This is because a relatively large amount of memory is normally required for video applications. In consumer applications and other applications where cost is a concern, it is desirable to minimize the amount of memory required to implement an image processing system or device.
A known MPEG2 decoder 100 followed by a conventional interlace to progressive (I-P) scan converter 110 is illustrated in FIG. 1. The (I-P) scan converter 110 operates to convert interlaced image data into progressive image data. Switching between intra-field and inter-field interpolation is performed in the system of FIG. 1 as a function of detected motion in the image represented by the video data being decoded.
A frame memory includes sufficient memory to store data representing an entire frame. Known MPEG2 decoders such as the decoder 100 normally use three frame memories, a first anchor frame memory 102, a second anchor frame memory 104 and a B-frame buffer 106 to decode MPEG 2 data as shown in FIG. 1. The conventional I-P converter 110 which follows the MEPG2 decoder in FIG. 1 uses an additional frame memory 112 for interpolation and motion detection purposes. Accordingly, a conventional MPEG2 decoder 100 followed by an I-P converter 110 normally requires a total of four frame memories.
In order to reduce the cost of video systems which perform decoding and conversion operations, there is a need for methods and apparatus which allow for a reduction in the amount of memory required to implement such systems. It is desirable that any new methods and apparatus be suitable for implementation in computer systems as well as televisions sets, set top boxes and other video applications.